The PDGF proteins and their receptors (PDGFRs) are involved in regulation of cell proliferation, survival and migration of several cell types. The VEGF proteins and their receptors (VEGFRs) play important roles in both vasculogenesis, the development of the embryonic vasculature from early differentiating endothelial cells, and angiogenesis, the process of forming new blood vessels from pre-existing ones [Risau et al., Dev Biol 125:441-450 (1988); Zachary, Intl J Biochem Cell Bio 30:1169-1174 (1998); Neufeld et al., FASEB J 13:9-22 (1999); Ferrara, J Mol Med 77:527-543 (1999)]. Both processes depend on the tightly controlled endothelial cell proliferation, migration, differentiation, and survival. Dysfunction of the endothelial cell regulatory system is a key feature of cancer and several diseases associated with abnormal angiogenesis, such as proliferative retinopathies, age-related muscular degeneration, rheumatoid arthritis, and psoriasis. Understanding of the specific biological function of the key players involved in regulating endothelial cells will lead to more effective therapeutic applications to treat such diseases [Zachary, Intl J Biochem Cell Bio 30:1169-1174 (1998); Neufeld et al., FASEB J 13:9-22 (1999); Ferrara, J Mol Med 77:527-543 (1999)].
The PDGF/VEGF Family
The PDGF/VEGF family of growth factors includes at least the following members: PDGF-A (see e.g., GenBank Acc. No. X06374), PDGF-B (see e.g., GenBank Acc. No. M12783), VEGF (see e.g., GenBank Acc. No. Q16889 referred to herein for clarity as VEGF-A or by particular isoform), PIGF (see e.g., GenBank Acc. No. X54936 placental growth factor), VEGF-B (see e.g., GenBank Acc. No. U48801; also known as VEGF-related factor (VRF)), VEGF-C (see e.g., GenBank Acc. No. X94216; also known as VEGF related protein (VRP)), VEGF-D (also known as c-fos-induced growth factor (FIGF); see e.g., Genbank Acc. No. AJ00185), VEGF-E (also known as NZ7 VEGF or OV NZ7; see e.g., GenBank Acc. No. S67522), NZ2 VEGF (also known as OV NZ2; see e.g., GenBank Acc. No. S67520), D1701 VEGF-like protein (see e.g., GenBank Acc. No. AF106020; Meyer et al., EMBO J 18:363-374), and NZ10 VEGF-like protein (described in International Patent Application PCT/US99/25869) [Stacker and Achen, Growth Factors 17:1-11 (1999); Neufeld et al., FASEB J 13:9-22 (1999); Ferrara, J Mol Med 77:527-543 (1999)].
Members of the PDGF/VEGF family are characterized by a number of structural motifs including a conserved PDGF motif defined by the sequence: P-[PS]-C-V-X(3)-R-C-[GSTA]-G-C-C (SEQ ID NO: 1200). The brackets indicate that this position within the polypeptide can be any one of the amino acids contained within the brackets. The number contained within the parentheses indicates the number of amino acids that separate the “V” and “R” residues. This conserved motif falls within a large domain of 70-150 amino acids defined in part by eight highly conserved cysteine residues that form inter- and intramolecular disulfide bonds. This domain forms a cysteine knot motif composed of two disulfide bonds which form a covalently linked ring structure between two adjacent β strands, and a third disulfide bond that penetrates the ring [see for example, FIG. 1 in Muller et al., Structure 5:1325-1338 (1997)], similar to that found in other cysteine knot growth factors, e.g., transforming growth factor-β (TGF-β). The amino acid sequence of all known PDGF/VEGF proteins, with the exception of VEGF-E, contains the PDGF domain. The PDGF/VEGF family proteins are predominantly secreted glycoproteins that form either disulfide-linked or non-covalently bound homo- or heterodimers whose subunits are arranged in an anti-parallel manner [Stacker and Achen, Growth Factors 17:1-11 (1999); Muller et al., Structure 5:1325-1338 (1997)].
The PDGF Subfamily
The PDGFs regulate cell proliferation, cell survival and chemotaxis of many cell types in vitro (reviewed in [Heldin et al., Biochimica et Biophysica Acta 1378:F79-113 (1998)]. The two chains that make up PDGF, PDGF-A and PDGF-B, can homo- or heterodimerize producing three different isoforms: PDGF-AA, PDGF-AB, or PDGF-BB. PDGF-A is only able to bind the PDGF α-receptor (PDGFR-α), whereas PDGF-B can bind both the PDGF-α and a second PDGF receptor (PDGF-α). In vivo, the PDGF proteins exert their effects in a paracrine manner since they often are expressed in epithelial (PDGF-A) or endothelial (PDGF-B) cells in close apposition to the PDGF receptor-expressing mesenchyme (reviewed in Ataliotis et al., Int Rev Cytology 172:95-127 (1997)]. Overexpression of the PDGFs has been observed in several pathological conditions, including malignancies, atherosclerosis, and fibroproliferative diseases. In tumor cells and cell lines grown in vitro, co-expression of the PDGFs and PDGF receptors generates autocrine loops, which are important for cellular transformation [Betsholtz et al., Cell 39:447-57 (1984); Keating et al., Science 239:914-6 (1988)].
The importance of the PDGFs as regulators of cell proliferation and cell survival is well illustrated by recent gene targeting studies in mice. Homozygous null mutations for either PDGF-A or PDGF-B are lethal in mice. Approximately 50% of the homozygous PDGF-A deficient mice have an early lethal phenotype, while the surviving animals have a complex postnatal phenotype with lung emphysema due to improper alveolar septum formation, and a dermal phenotype characterized by thin dermis, misshapen hair follicles, and thin hair. PDGF-A is also required for normal development of oligodendrocytes and subsequent myelination of the central nervous system. The PDGF-B deficient mice develop renal, hematological and cardiovascular abnormalities; where the renal and cardiovascular defects, at least in part, are due to the lack of proper recruitment of mural cells (vascular smooth muscle cells, pericytes or mesangial cells) to blood vessels.
The VEGF Subfamily
The VEGF subfamily is composed of PDGF/VEGF members which share a VEGF homology domain (VHD) characterized by the sequence: C-X(22-24)-P-[PSR]-C-V-X(3)-R-C-[GSTA]-G-C-C-X(6)-C-X(32-41)-C (SEQ ID NO: 1201). The VHD domain, determined through analysis of the VEGF subfamily members, comprises the PDGF motif but is more specific.
VEGF-A was originally purified from several sources on the basis of its mitogenic activity toward endothelial cells, and also by its ability to induce microvascular permeability, hence it is also called vascular permeability factor (VPF). VEGF-A has subsequently been shown to induce a number of biological processes including the mobilization of intracellular calcium, the induction of plasminogen activator and plasminogen activator inhibitor-1 synthesis, promotion of monocyte migration in vitro, induction of antiapoptotic protein expression in human endothelial cells, induction of fenestrations in endothelial cells, promotion of cell adhesion molecule expression in endothelial cells and induction of nitric oxide mediated vasodilation and hypotension [Ferrara, J Mol Med 77: 527-543 (1999); Neufeld et al., FASEB J 13: 9-22 (1999); Zachary, Intl J Biochem Cell Bio 30: 1169-1174 (1998)].
VEGF-A is a secreted, disulfide-linked homodimeric glycoprotein composed of 23 kD subunits. Five human VEGF-A isoforms of 121, 145, 165, 189 or 206 amino acids in length (VEGF121-206), encoded by distinct mRNA splice variants, have been described, all of which are capable of stimulating mitogenesis in endothelial cells. However, each isoform differs in biological activity, receptor specificity, and affinity for cell surface- and extracellular matrix-associated heparan-sulfate proteoglycans, which behave as low affinity receptors for VEGF-A. VEGF121 does not bind to either heparin or heparan-sulfate; VEGF145 and VEGF165 (GenBank Acc. No. M32977) are both capable of binding to heparin; and VEGF189 and VEGF206 show the strongest affinity for heparin and heparan-sulfates. VEGF121, VEGF145, and VEGF165 are secreted in a soluble form, although most of VEGF165 is confined to cell surface and extracellular matrix proteoglycans, whereas VEGF189 and VEGF206 remain associated with extracellular matrix. Both VEGF189 and VEGF206 can be released by treatment with heparin or heparinase, indicating that these isoforms are bound to extracellular matrix via proteoglycans. Cell-bound VEGF189 can also be cleaved by proteases such as plasmin, resulting in release of an active soluble VEGF110. Most tissues that express VEGF are observed to express several VEGF isoforms simultaneously, although VEGF121 and VEGF165 are the predominant forms, whereas VEGF206 is rarely detected [Ferrara, J Mol Med 77:527-543 (1999)]. VEGF145 differs in that it is primarily expressed in cells derived from reproductive organs [Neufeld et al., FASEB J 13:9-22 (1999)].
The pattern of VEGF-A expression suggests its involvement in the development and maintenance of the normal vascular system, and in angiogenesis associated with tumor growth and other pathological conditions such as rheumatoid arthritis. VEGF-A is expressed in embryonic tissues associated with the developing vascular system, and is secreted by numerous tumor cell lines. Analysis of mice in which VEGF-A was knocked out by targeted gene disruption indicate that VEGF-A is critical for survival, and that the development of the cardiovascular system is highly sensitive to VEGF-A concentration gradients. Mice lacking a single copy of VEGF-A die between day 11 and 12 of gestation. These embryos show impaired growth and several developmental abnormalities including defects in the developing cardiovasculature. VEGF-A is also required post-natally for growth, organ development, regulation of growth plate morphogenesis and endochondral bone formation. The requirement for VEGF-A decreases with age, especially after the fourth postnatal week. In mature animals, VEGF-A is required primarily for active angiogenesis in processes such as wound healing and the development of the corpus luteum. [Neufeld et al., FASEB J 13:9-22 (1999); Ferrara, J Mol Med 77:527-543 (1999)]. VEGF-A expression is influenced primarily by hypoxia and a number of hormones and cytokines including epidermal growth factor (EGF), TGF-β, and various interleukins. Regulation occurs transcriptionally and also post-transcriptionally such as by increased mRNA stability [Ferrara, J Mol Med 77:527-543 (1999)].
PlGF, a second member of the VEGF subfamily, is generally a poor stimulator of angiogenesis and endothelial cell proliferation in comparison to VEGF-A, and the in vivo role of PlGF is not well understood. Three isoforms of PlGF produced by alternative mRNA splicing have been described [Hauser et al., Growth Factors 9:259-268 (1993); Maglione et al., Oncogene 8:925-931 (1993)]. PlGF forms both disulfide-liked homodimers and heterodimers with VEGF-A. The PlGF-VEGF-A heterodimers are more effective at inducing endothelial cell proliferation and angiogenesis than PlGF homodimers. PlGF is primarily expressed in the placenta, and is also co-expressed with VEGF-A during early embryogenesis in the trophoblastic giant cells of the parietal yolk sac [Stacker and Achen, Growth Factors 17:1-11 (1999)].
VEGF-B, described in detail in International Patent Publication No. WO 96/26736 and U.S. Pat. Nos. 5,840,693 and 5,607,918, shares approximately 44% amino acid identity with VEGF-A. Although the biological functions of VEGF-B in vivo remain incompletely understood, it has been shown to have angiogenic properties, and may also be involved in cell adhesion and migration, and in regulating the degradation of extracellular matrix. It is expressed as two isoforms of 167 and 186 amino acid residues generated by alternative splicing. VEGF-B167 is associated with the cell surface or extracellular matrix via a heparin-binding domain, whereas VEGF-B186 is secreted. Both VEGF-B167 and VEGF-B186 can form disulfide-linked homodimers or heterodimers with VEGF-A. The association to the cell surface of VEGF165-VEGF-B167 heterodimers appears to be determined by the VEGF-B component, suggesting that heterodimerization may be important for sequestering VEGF-A. VEGF-B is expressed primarily in embryonic and adult cardiac and skeletal muscle tissues [Joukov et al., J Cell Physiol 173:211-215 (1997); Stacker and Achen, Growth Factors 17:1-11 (1999)]. Mice lacking VEGF-B survive but have smaller hearts, dysfunctional coronary vasculature, and exhibit impaired recovery from cardiac ischemia [Bellomo et al., Circ Res 2000;E29-E35].
A fourth member of the VEGF subfamily, VEGF-C, comprises a VHD that is approximately 30% identical at the amino acid level to VEGF-A. VEGF-C is originally expressed as a larger precursor protein, prepro-VEGF-C, having extensive amino- and carboxy-terminal peptide sequences flanking the VHD, with the C-terminal peptide containing tandemly repeated cysteine residues in a motif typical of Balbiani ring 3 protein. Prepro-VEGF-C undergoes extensive proteolytic maturation involving the successive cleavage of a signal peptide, the C-terminal pro-peptide, and the N-terminal pro-peptide. Secreted VEGF-C protein consists of a non-covalently-linked homodimer, in which each monomer contains the VHD. The intermediate forms of VEGF-C produced by partial proteolytic processing show increasing affinity for the VEGFR-3 receptor, and the mature protein is also able to bind to the VEGFR-2 receptor. [Joikov et al., EMBO J., 16:(13):3898-3911 (1997).] It has also been demonstrated that a mutant VEGF-C, in which a single cysteine at position 156 is either substituted by another amino acid or deleted, loses the ability to bind VEGFR-2 but remains capable of binding and activating VEGFR-3 [International Patent Publication No. WO 98/33917]. In mouse embryos, VEGF-C mRNA is expressed primarily in the allantois, jugular area, and the metanephros. [Joukov et al., J Cell Physiol 173:211-215 (1997)]. VEGF-C is involved in the regulation of lymphatic angiogenesis: when VEGF-C was overexpressed in the skin of transgenic mice, a hyperplastic lymphatic vessel network was observed, suggesting that VEGF-C induces lymphatic growth [Jeltsch et al., Science, 276:1423-1425 (1997)]. Continued expression of VEGF-C in the adult also indicates a role in maintenance of differentiated lymphatic endothelium [Ferrara, J Mol Med 77:527-543 (1999)]. VEGF-C also shows angiogenic properties: it can stimulate migration of bovine capillary endothelial (BCE) cells in collagen and promote growth of human endothelial cells [see, e.g., International Patent Publication No. WO 98/33917, incorporated herein by reference].
VEGF-D is structurally and functionally most closely related to VEGF-C [see International Patent Publ. No. WO 98/07832, incorporated herein by reference]. Like VEGF-C, VEGF-D is initially expressed as a prepro-peptide that undergoes N-terminal and C-terminal proteolytic processing, and forms non-covalently linked dimers. VEGF-D stimulates mitogenic responses in endothelial cells in vitro. During embryogenesis, VEGF-D is expressed in a complex temporal and spatial pattern, and its expression persists in the heart, lung, and skeletal muscles in adults. Isolation of a biologically active fragment of VEGF-D designated VEGF-DΔNΔC, is described in International Patent Publication No. WO 98/07832, incorporated herein by reference. VEGF-DΔNΔC consists of amino acid residues 93 to 201 of VEGF-D linked to the affinity tag peptide FLAG®.
Four additional members of the VEGF subfamily have been identified in poxviruses, which infect humans, sheep and goats. The orf virus-encoded VEGF-E and NZ2 VEGF are potent mitogens and permeability enhancing factors. Both show approximately 25% amino acid identity to mammalian VEGF-A, and are expressed as disulfide-liked homodimers. Infection by these viruses is characterized by pustular dermititis which may involve endothelial cell proliferation and vascular permeability induced by these viral VEGF proteins. [Ferrara, J Mol Med 77:527-543 (1999); Stacker and Achen, Growth Factors 17:1-11 (1999)]. VEGF-like proteins have also been identified from two additional strains of the orf virus, D1701 [GenBank Acc. No. AF106020; described in Meyer et al., EMBO J 18:363-374 (1999)] and NZ10 [described in International Patent Application PCT/US99/25869, incorporated herein by reference]. These viral VEGF-like proteins have been shown to bind VEGFR-2 present on host endothelium, and this binding is important for development of infection and viral induction of angiogenesis [Meyer et al., EMBO J 18:363-374 (1999); International Patent Application PCT/US99/25869].
PDGF/VEGF Receptors
Seven cell surface receptors that interact with PDGF/VEGF family members have been identified. These include PDGFR-α (see e.g., GenBank Acc. No. NM006206), PDGFR-β (see e.g., GenBank Acc. No. NM002609), VEGFR-1/Flt-1 (fms-like tyrosine kinase-1; GenBank Acc. No. X51602; De Vries et al., Science 255:989-991 (1992)); VEGFR-2/KDR/Flk-1 (kinase insert domain containing receptor/fetal liver kinase-1; GenBank Acc. Nos. X59397 (Flk-1) and L04947 (KDR); Terman et al., Biochem Biophys Res Comm 187:1579-1586 (1992); Matthews et al., Proc Natl Acad Sci USA 88:9026-9030 (1991)); VEGFR-3/Flt4 (fms-like tyrosine kinase 4; U.S. Pat. No. 5,776,755 and GenBank Acc. No. X68203 and S66407; Pajusola et al., Oncogene 9:3545-3555 (1994)), neuropilin-1 (Gen Bank Acc. No. NM003873), and neuropilin-2 (Gen Bank Acc. No. NM003872). The two PDGF receptors mediate signaling of PDGFs as described above. VEGF121, VEGF165, VEGF-B, PlGF-1 and PlGF-2 bind VEGF-R1; VEGF121, VEGF145, VEGF165, VEGF-C, VEGF-D, VEGF-E, and NZ2 VEGF bind VEGF-R2; VEGF-C and VEGF-D bind VEGFR-3; VEGF165, PlGF-2, and NZ2 VEGF bind neuropilin-1; and VEGF165 binds neuropilin-2. [Neufeld et al., FASEB J 13:9-22 (1999); Stacker and Achen, Growth Factors 17:1-11 (1999); Ortega et al., Fron Biosci 4:141-152 (1999); Zachary, Intl J Biochem Cell Bio 30:1169-1174 (1998); Petrova et al., Exp Cell Res 253:117-130 (1999)].
The PDGF receptors are protein tyrosine kinase receptors (PTKs) that contain five immunoglobulin-like loops in their extracellular domains. VEGFR-1, VEGFR-2, and VEGFR-3 comprise a subgroup of the PDGF subfamily of PTKs, distinguished by the presence of seven Ig domains in their extracellular domain and a split kinase domain in the cytoplasmic region. Both neuropilin-1 and neuropilin-2 are non-PTK VEGF receptors. NP-1 has an extracellular portion includes a MAM domain; regions of homology to coagulation factors V and VIII, MFGPs and the DDR tyrosine kinase; and two CUB-like domains.
Several of the VEGF receptors are expressed as more than one isoform. A soluble isoform of VEGFR-1 lacking the seventh Ig-like loop, transmembrane domain, and the cytoplasmic region is expressed in human umbilical vein endothelial cells. This VEGFR-1 isoform binds VEGF-A with high affinity and is capable of preventing VEGF-A-induced mitogenic responses [Ferrara, J Mol Med 77:527-543 (1999); Zachary, Intl J Biochem Cell Bio 30:1169-1174 (1998)]. A C-terminal truncated from of VEGFR-2 has also been reported [Zachary, Intl J Biochem Cell Bio 30:1169-1174 (1998)]. In humans, there are two isoforms of the VEGFR-3 protein which differ in the length of their C-terminal ends. Studies suggest that the longer isoform is responsible for most of the biological properties of VEGFR-3.
The receptors for the PDGFs, PDGF α-receptor (PDGFR-α) and the β-receptor (PDGFR-β), are expressed by many in vitro grown cell lines, and they are mainly expressed by mesenchymal cells in vivo (reviewed in [Raines et al., Peptide growth factors and their receptors, Heidelberg, Springer-Verlag (1990)]. As mentioned above, PDGF-B binds both PDGFRs, while PDGF-A selectively binds PDGFR-α.
Gene targeting studies in mice have revealed distinct physiological roles for the PDGF receptors despite the overlapping ligand specificities of the PDGFRs [Rosenkranz et al., Growth Factors 16:201-16 (1999)]. Homozygous null mutations for either of the two PDGF receptors are lethal. PDGFR-α deficient mice die during embryogenesis at e10, and show incomplete cephalic closure, impaired neural crest development, cardiovascular defects, skeletal defects, and odemas. The PDGFR-β deficient mice develop similar phenotypes to animals deficient in PDGF-B, that are characterized by renal, hematological and cardiovascular abnormalities; where the renal and cardiovascular defects, at least in part, are due to the lack of proper recruitment of mural cells (vascular smooth muscle cells, pericytes or mesangial cells) to blood vessels.
The expression of VEGFR-1 occurs mainly in vascular endothelial cells, although some may be present on monocytes, trophoblast cells, and renal mesangial cells [Neufeld et al., FASEB J 13:9-22 (1999)]. High levels of VEGFR-1 mRNA are also detected in adult organs, suggesting that VEGFR-1 has a function in quiescent endothelium of mature vessels not related to cell growth. VEGFR-1−/− mice die in utero between day 8.5 and 9.5. Although endothelial cells developed in these animals, the formation of functional blood vessels was severely impaired, suggesting that VEGFR-1 may be involved in cell-cell or cell-matrix interactions associated with cell migration. Recently, it has been demonstrated that mice expressing a mutated VEGFR-1 in which only the tyrosine kinase domain was missing show normal angiogenesis and survival, suggesting that the signaling capability of VEGFR-1 is not essential. [Neufeld et al., FASEB J 13:9-22 (1999); Ferrara, J Mol Med 77:527-543 (1999)].
VEGFR-2 expression is similar to that of VEGFR-1 in that it is broadly expressed in the vascular endothelium, but it is also present in hematopoietic stem cells, megakaryocytes, and retinal progenitor cells [Neufeld et al., FASEB J 13:9-22 (1999)]. Although the expression pattern of VEGFR-1 and VEGFR-2 overlap extensively, evidence suggests that, in most cell types, VEGFR-2 is the major receptor through which most of the VEGFs exert their biological activities. Examination of mouse embryos deficient in VEGFR-2 further indicate that this receptor is required for both endothelial cell differentiation and the development of hematopoietic cells [Joukov et al., J Cell Physiol 173:211-215 (1997)].
VEGFR-3 is expressed broadly in endothelial cells during early embryogenesis. During later stages of development, the expression of VEGFR-3 becomes restricted to developing lymphatic vessels [Kaipainen, A., et al., Proc. Natl. Acad. Sci. USA, 92: 3566-3570 (1995)]. In adults, the lymphatic endothelia and some high endothelial venules express VEGFR-3, and increased expression occurs in lymphatic sinuses in metastatic lymph nodes and in lymphangioma. VEGFR-3 is also expressed in a subset of CD34+ hematopoietic cells which may mediate the myelopoietic activity of VEGF-C demonstrated by overexpression studies [WO 98/33917]. Targeted disruption of the VEGFR-3 gene in mouse embryos leads to failure of the remodeling of the primary vascular network, and death after embryonic day 9.5 [Dumont et al., Science, 282: 946-949 (1998)]. These studies suggest an essential role for VEGFR-3 in the development of the embryonic vasculature, and also during lymphangiogenesis.
Structural analyses of the VEGF receptors indicate that the VEGF-A binding site on VEGFR-1 and VEGFR-2 is located in the second and third Ig-like loops. Similarly, the VEGF-C and VEGF-D binding sites on VEGFR-2 and VEGFR-3 are also contained within the second Ig-loop [Taipale et al., Curr Top Microbiol Immunol 237:85-96 (1999)]. The second Ig-like loop also confers ligand specificity as shown by domain swapping experiments [Ferrara, J Mol Med 77:527-543 (1999)]. Receptor-ligand studies indicate that dimers formed by the VEGF family proteins are capable of binding two VEGF receptor molecules, thereby dimerizing VEGF receptors. The fourth Ig-like loop on VEGFR-1, and also possibly on VEGFR-2, acts as the receptor dimerization domain that links two receptor molecules upon binding of the receptors to a ligand dimer [Ferrara, J Mol Med 77:527-543(1999)]. Although the regions of VEGF-A that bind VEGFR-1 and VEGFR-2 overlap to a large extent, studies have revealed two separate domains within VEGF-A that interact with either VEGFR-1 or VEGFR-2, as well as specific amino acid residues within these domains that are critical for ligand-receptor interactions. Mutations within either VEGF receptor-specific domain that specifically prevent binding to one particular VEGF receptor have also been recovered [Neufeld et al., FASEB J 13:9-22 (1999)].
VEGFR-1 and VEGFR-2 are structurally similar, share common ligands (VEGF121, and VEGF165), and exhibit similar expression patterns during development. However, the signals mediated through VEGFR-1 and VEGFR-2 by the same ligand appear to be slightly different. VEGFR-2 has been shown to undergo autophosphorylation in response to VEGF-A, but phosphorylation of VEGFR-1 under identical conditions was barely detectable. VEGFR-2 mediated signals cause striking changes in the morphology, actin reorganization, and membrane ruffling of porcine aortic endothelial cells recombinantly overexpressing this receptor. In these cells, VEGFR-2 also mediated ligand-induced chemotaxis and mitogenicity; whereas VEGFR-1-transfected cells lacked mitogenic responses to VEGF-A. Mutations in VEGF-A that disrupt binding to VEGFR-2 fail to induce proliferation of endothelial cells, whereas VEGF-A mutants that are deficient in binding VEGFR-1 are still capable of promoting endothelial proliferation. Similarly, VEGF stimulation of cells expressing only VEGFR-2 leads to a mitogenic response whereas comparable stimulation of cells expressing only VEGFR-1 also results in cell migration, but does not induce cell proliferation. In addition, phosphoproteins co-precipitating with VEGFR-1 and VEGFR-2 are distinct, suggesting that different signaling molecules interact with receptor-specific intracellular sequences.
The emerging hypothesis is that the primary function of VEGFR-1 in angiogenesis may be to negatively regulate the activity of VEGF-A by binding it and thus preventing its interaction with VEGFR-2, whereas VEGFR-2 is thought to be the main transducer of VEGF-A signals in endothelial cells. In support of this hypothesis, mice deficient in VEGFR-1 die as embryos while mice expressing a VEGFR-1 receptor capable of binding VEGF-A but lacking the tyrosine kinase domain survive and do not exhibit abnormal embryonic development or angiogenesis. In addition, analyses of VEGF-A mutants that bind only VEGFR-2 show that they retain the ability to induce mitogenic responses in endothelial cells. However, VEGF-mediated migration of monocytes is dependent on VEGFR-1, indicating that signaling through this receptor is important for at least one biological function. In addition, the ability of VEGF-A to prevent the maturation of dendritic cells is also associated with VEGFR-1 signaling, suggesting that VEGFR-1 may function in cell types other than endothelial cells. [Ferrara, J Mol Med 77:527-543 (1999); Zachary, Intl J Biochem Cell Bio 30:1169-1174 (1998)].
Neuropilin-1 was originally cloned as a receptor for the collapsin/semaphorin family of proteins involved in axon guidance [Stacker and Achen, Growth Factors 17:1-11 (1999)]. It is expressed in both endothelia and specific subsets of neurons during embryogenesis, and it thought to be involved in coordinating the developing. neuronal and vascular system. Although activation of neuropilin-1 does not appear to elicit biological responses in the absence of the VEGF family tyrosine-kinase receptors, their presence on cells leads to more efficient binding of VEGF165 and VEGFR-2 mediated responses. [Neufeld et al., FASEB J 13:9-22 (1999)] Mice lacking neuropilin-1 show abnormalities in the developing embryonic cardiovascular system. [Neufeld et al., FASEB J 13:9-22 (1999)]
Neuropilin-2 was identified by expression cloning and is a collapsin/semaphorin receptor closely related to neuropilin-1. Neuropilin-2 is an isoform-specific VEGF receptor in that it only binds VEGF165. Like neuropilin-1, neuropilin-2 is expressed in both endothelia and specific neurons, and is not predicted to function independently due to its relatively short intracellular domain. The function of neuropilin-2 in vascular development is unknown [Neufeld et al., FASEB J 13:9-22 (1999); WO 99/30157].
Therapeutic Applications for VEGF Polypeptides and Antagonists
The discovery of VEGF-A as a key regulator of vascular development has spurred active research using VEGF-based therapeutic angiogenesis in cardiovascular medicine, as well as for treating diseases characterized by pathological angiogenesis with VEGF antagonists. Subsequent identification of additional VEGF family proteins and their roles in vascularization have also led to the development of therapies based on these growth factors [Ferrara and Alitalo, Nature Med 5:1359-1364 (1999)]. Animal studies of hindlimb ischemia, and myocardial ischemia using VEGF-A or VEGF-C, delivered by administration of recombinant protein or gene transfer using naked DNA or adenoviral vectors, implicate these molecules in promoting vascularization and increasing coronary blood flow. These promising results have led to clinical trials in which patients with limb ischemia were treated by arterial or intramuscular gene transfer of naked DNA encoding VEGF165. Patients with myocardial ischemia or Burger's disease (thromboangiitis obliterans) were also injected locally with VEGF165 plasmid DNA. Although these trials were not placebo-controlled, the patients showed clinical improvement and evidence of angiogenesis in ischemic tissues. Trials using gene transfer of VEGF-C naked DNA or gene therapy with VEGF121 using adenoviral vectors to treat patients with myocardial ischemia are currently in Phase I [Ferrara, J Mol Med 77:527-543 (1999); Neufeld et al., FASEB J 13:9-22 (1999); Ferrara and Alitalo, Nature Med 5:1359-1364 (1999)]. The therapeutic effects of administering recombinant VEGF-A protein are also being tested in ongoing clinical trials. Results from a Phase I study of patients with coronary ischemia treated with intracoronary infusion of recombinant VEGF165 show evidence of improved perfusion and collateralization. However, in the subsequent Phase II study, the patients did not show significant improvement over the placebo-controlled group. Other potential therapeutic uses for VEGF growth factors include using VEGF-C to promote lymphangiogenesis in patients whose axillary lymph nodes were removed during breast carcinoma surgery. Therapies using combinations of growth factors to promote vascularization in tissues may also prove to be preferable in treating certain diseases [Ferrara and Alitalo, Nature Med 5:1359-1364 (1999)].
Therapies based on inhibiting the activity of VEGF growth factors are being tested to treat disease states characterized by pathological angiogenesis. VEGF expression is upregulated in most human tumors including primary breast cancer and gastric carcinoma. Studies in mice indicate that tumor-associated angiogenesis and growth of the tumor cells can be inhibited by treating the animals with monoclonal antibodies against VEGF-A. Further animal studies showed that expression of a dominant negative VEGFR-2 mutant that prevents signaling through this receptor, or administration of recombinant VEGFR-1 or VEGFR-2 mutants, which only contain the extracellular portion of these receptors, suppresses growth of several tumor cell lines. These encouraging results led to clinical trials using humanized high affinity monoclonal antibodies against VEGF-A (rhuMAb VEGF) as VEGF-A inhibitors. Phase II studies using rhuMAb VEGF to treat non-small cell lung carcinoma, colorectal carcinoma, breast, and renal cell carcinoma are currently ongoing. Compounds targeting inhibition of VEGF-C activity are also being tested for therapeutic uses in cancer patients: small molecule inhibitors of VEGF-C are in Phase II trials, and monoclonal antibodies against VEGF-C are entering clinical trials.
Retinopathy associated with diabetes mellitus, occlusion of central retinal vein or prematurity has been correlated with increased levels of VEGF-A. Animal studies using monoclonal antibodies against VEGF-A or soluble VEGFR-1 or VEGFR-2 mutants containing only the extracellular domain fused to immunoglobulin γFc domain show suppression of retinal angiogenesis. VEGF-A is also detected in age-related macular degeneration (AMD), and its expression is thought to be the cause of neovascularization in this disease. Intravitreal delivery of recombinant humanized anti-VEGF-A Fab antibody fragment or injection of 2′-fluoropyrimidine RNA oligonucleotide ligands (aptamers) to treat AMD are currently in clinical trials. Compounds that inhibit the activity of VEGF growth factors may also be used to treat other disease states involving abnormal angiogenesis. These include ischemic-reperfusion related brain edema and injury, conditions associated with ovarian hyperplasia and hypervascularity such as the polycystic ovary syndrome, endometriosis, and ovarian hyperstimulation syndrome [Ferrara and Alitalo, Nature Med 5:1359-1364 (1999)].
From the foregoing discussion, it will be apparent that the VEGF family of growth factors, and inhibitors thereof, have tremendous potential as therapeutics. For example, such growth factors and inhibitors are useful to promote or inhibit angiogenesis where needed, such as in the treatment of ischemic disorders, the promotion of wound healing, or the inhibition or elimination of neoplastic disorders that are angiogenesis-dependent. However, the various naturally-occurring members of this growth factor family often bind multiple receptors, and the various known receptors are expressed on multiple cell types and have expression patterns that may vary depending on stage of development and the presence or absence of pathological conditions. The biological effects of any particular growth factor may be receptor-dependent, isoform dependent, and cell-type dependent. A desirable therapeutic effect mediated through one receptor may be accompanied by undesirable side-effects mediated through another receptor. Alternatively, a desirable therapeutic effect might be enhanced through stimulation of multiple receptors that cannot be stimulated with any single known growth factor that occurs in nature. Therefore, a need exists for novel peptide growth factors with their own unique profile of receptor binding and receptor-stimulating or receptor-inhibiting activities.